The process of growth

Growth is seldom random. Rather, it occurs according to a plan that eventually determines the size and shape of the individual. Growth may be restricted to special regions of the organism, such as the layers of cells that divide and increase in size near the tip of the plant shoot. Or the cells engaged in growth may be widely distributed throughout the body of the organism, as in the human embryo. In the latter case, the rates of cell division and of the increase in cell size differ in different parts. That the pattern of growth is predetermined and regular in plants and animals can be seen in the forms of adults. In some organisms, however, notably the slime molds, no regular pattern of growth occurs, and a formless cytoplasmic mass is the result.

The rate of growth of various components of an organism may have important consequences in its ability to adapt to the environment and hence may play a role in evolution. For instance, an increase in the rate of growth of fleshy parts of the fish fin would provide an opportunity for the fish to adapt more easily to terrestrial locomotory life than could a fish without this modified fin. Without disproportionate growth of the fin—ultimately resulting from random changes in the genetic material (mutations)—the evolution of limbs through natural selection might have been impossible.

Types of growth

In cells

The increase in size and changes in shape of a developing organism depend on the increase in the number and size of cells that make up the individual. Increase in cell number occurs by a precise cellular reproductive mechanism called mitosis. During mitosis the chromosomes bearing the genetic material are reproduced in the nucleus, and then the doubled chromosomes are precisely distributed to the two daughter cells, one of each chromosomal type going to each daughter cell. Each end of the dividing cell receives a complete set of chromosomes before the ends separate. In animal cells this is a pinching off (cytokinesis) of the cell membrane; in plant cells a new cellulose wall forms between the new cells.

During the period of cell life preceding the actual distribution of chromosomes, the mother cell often grows to twice its original size. Hence, a cycle consisting of cell growth and cell division is established. Cell growth—an increase in cytoplasmic mass, chromosome number, and cell surface—is followed by cell division, in which the cytoplasmic mass and chromosomes are distributed to the daughter cells. An increase in cytoplasmic mass does not always occur during cell-division cycles, however. During the early development of an embryo, for example, the original egg cell, usually a very large cell, undergoes repeated series of cell divisions without any intervening growth periods; as a result, the original egg cell divides into thousands of small cells. Only after the embryo can obtain food from its environment does the usual pattern of growth and mitosis occur.

The fact that most plant cells undergo extensive size increase unaccompanied by cell division is an important distinction between growth in plants and in animals. Daughter cells arising from cell division behind the tip of the plant root or shoot may undergo great increases in volume. This is accomplished through uptake of water by the cells; the water is stored in a central cavity called a vacuole. The intake of water produces a pressure that, in combination with other factors, pushes on the cellulose walls of the plant cells, thereby increasing the length, girth, and stiffness (turgor) of the cells and plant. In plants, much of the size increase occurs after cell division and results primarily from an increase in water content of the cells without much increase in dry weight.

The very young developing plant embryo has many cells distributed throughout its mass that undergo the cycle of growth and cell division. As soon as the positions of the root tip, shoot tip, and embryonic leaves become established, however, the potential for cell division becomes restricted to cells in certain regions called meristems. One meristematic centre lies just below the surface of the growing root; all increases in the number of cells of the primary root occur at this point. Some of the daughter cells remain at the elongating tip and continue to divide. Other daughter cells, which are left behind in the root, undergo the increase in length that enables the new root to push deeper into the soil. The same general plan is evident in the growing shoot of higher plants, in which a restricted meristematic region at the tip is responsible for the formation of the cells of the leaves and stem; cell elongation occurs behind this meristematic centre. The young seedling secondarily develops cells associated with the vascular strands of phloem and xylem—tissues that carry water to the leaves from the soil and sugar from the leaves to the rest of the plant. These cells can divide again, providing new cell material for development of a woody covering and for more elaborate vascular strands. Hence, the growth of higher plants—i.e., those aspects involving both the pattern of stems, leaves, and roots and the increase in bulk—results primarily from cell division at the meristem followed by a secondary increase in size because of water uptake. These activities occur throughout the period of plant growth.

The growth of animals is more restricted in time than is that of plants, but cell division is more generally distributed throughout the body of the organism. Although the rate of cell division differs in different regions, the capacity for cell division is widely distributed in the developing embryo. Increase in size is rapid during the embryonic period, continues at a reduced rate in juveniles, and thereafter is absent. Cell division and size increase continue, however, even after increase in total body size no longer occurs. Because these events are balanced by cell death, post-juvenile increase in cell number is primarily a replacement phenomenon. Height increase in mammals is limited by cessation of cell division and bone deposition in the long bones. The long juvenile period of growth in humans is unusual, most higher animals attaining mature size soon after the end of embryonic development. Some organ systems undergo little cell division and growth after birth; for instance, all of the germ cells (precursors of egg cells) of the female are formed by the time of birth. Similarly, all of the nerve cells of the brain are formed by the end of the embryonic period. Further increase in the size of the nervous system occurs by outgrowth of nerve fibres and deposition of a fatty insulation material along them. Although the greatest increase in size of nerve cells occurs, as in plant cells, after the cessation of cell division, the nerve fibre outgrowth in animals represents a true increase in the amount of cytoplasm and cell surface and not just an uptake of water.

Some organs retain the potential for growth and cell division throughout the life span of the animal. The liver, for example, continues to form new cells to replace senescent and dying ones. Although cell division and growth occur throughout the liver, other organs have a special population of cells, called stemcells, that retain the capacity for cell division. The cells that produce the circulating red cells of mammalian blood are found only in the marrow of the long bones. They form a permanent population of dividing cells, replacing the red cells that continuously die and disappear from the circulation.

The rates of both growth and cell division can vary widely in different body parts. This differential increase in size is a prime factor in defining the shape of an organism.

Normal and abnormal growth

When growth is not properly regulated, anomalies and tumours may result. If the increase in the number of liver cells is abnormal, for example, tumours of the liver, or hepatomas, may result. In fact, one feature of malignant tumours, or cancers, is the absence of the usual growth patterns and rates. The cells of malignant tumours, in addition to having abnormal growth rates, have altered adhesive properties, which enable them to detach easily from the tumour; in this way the cells may spread to other parts of the body (metastasize) and grow in unusual locations. It is the growth of tumours in places other than the organ of origin that usually causes the death of an organism. Tumours may vary widely in their growth rates. They may grow very rapidly or so slowly that the rate approaches that of normal cell division in adult tissues. Tumours are not only characterized by an increase in the rate of cell division but also by abnormal patterns of growth. The new cells formed in the tumour are not organized and incorporated into the structure of the organ and may form large nodules. These abnormal growths may present no medical problems (e.g., moles) or may cause disastrous effects, as is the case of the pressure on the brain caused by a tumorous mass of the meningeal covering of the brain.

Not all abnormal growths are tumours. If a tree is partially burned, cells below the bark produce a new covering for the exposed vascular strands. Growth may not be normal, and an obvious scar or growth of the new bark is apparent. Similarly, if the skin of a mammal is severely injured, the repair, although abnormal and imperfect, causes the organism no physiological difficulty. Many organisms possess the ability to regrow, or regenerate, with varying degrees of perfection, parts of the body that are lost or injured. Salamanders possess remarkable powers of regeneration, being able to form new eyes or a new limb if the original is lost. Lizards can regenerate a new tail; even humans can regenerate parts of the liver. The reasons for the differences in regenerative powers in different animals remain a fascinating mystery of great practical importance. When regeneration does occur, some specialized cells usually lose their specialized characteristics and enter a period of an increased rate of cell division; subsequently, the new cells respecialize into the tissues of the original body part. Plants whose tops are lost as in pruning can also sometimes form new meristematic centres from dormant tissues and produce new shoots.

Compensatory growth

Many organs of animals occur in pairs, and if one is lost the remaining member increases in size, as if responding to the demands of increased use. If one of the two kidneys of a human is removed, for example, the other increases in size. This is called a compensatory reaction and may occur either by some increase in cell size (hypertrophy), by an increase in the rate of cell division (hyperplasia), or both. Although an increase in cell number is primarily responsible for the compensatory reaction of the kidney, the number of individual filtration units (glomeruli) does not increase. Hence, cell division increases the size of glomeruli but not the total number. Some of the most striking examples of increases in cell size in animals take place during stimulation of endocrine organs, which secrete regulatory substances called hormones; when the thyroid gland is stimulated, for example, the individual cells of the gland may increase dramatically in size.

Factors that regulate growth

Environmental factors

The environment in which an organism lives plays an important role in modifying the rate and extent of growth. Environmental factors may be either physical (e.g., temperature, radiant energy, and atmospheric pressure) or chemical. Organisms and the cells of which they are composed are extremely sensitive to temperature changes; as the temperature decreases, the biochemical reactions necessary for life occur more slowly. A lowering of the temperature by 10° C (18° F) slows metabolism at least twofold and often more.

The width of trees increases partly by cell division and enlargement of secondary meristematic tissue below the bark. During the cold of winter, cell division and enlargement may cease completely; but during the spring renewed growth occurs. This intermittent growth is influenced by temperature, light, and water. The amount of growth may decrease considerably if the spring is cold, if day length is changed by obstructions blocking the sunlight, or if a drought occurs. In fact, the width of the growth rings visible on the surface of the cut tree trunk provides a partial history of climatic conditions, the spacing of the growth rings of different size having been correlated with known periods of drought and cold to provide reliable archaeological dating of various structures, as in the timbers used in Indian pueblos in the southwestern United States.

Temperature also affects both warm- and cold-blooded animals. Many warm-blooded (e.g., bears) and cold-blooded (e.g., frogs) vertebrates cease growing during the cold winter and simply enter an inactive or dormant state, which is characterized by a very low rate of metabolism. In animals that do not become dormant, increased demands for food consumption occur during cold periods to provide energy to maintain body temperature; this utilization of food energy may limit the energy available for size increase if food is in short supply.

Because atmospheric pressure is relatively constant except in the mountains, it probably is of little importance in growth regulation. Increases in pressure in the ocean’s depths may be significant, however, since it is known that increases in hydrostatic pressure interfere with cell division. Tissues of deep-sea fishes must have become adapted to such pressure effects, which have been little studied thus far. Movements of the terrestrial atmosphere—winds—may affect growth patterns in trees and shrubs, as is evident in the exotic shapes of certain conifers that grow along coastlines exposed to strong prevailing winds.

Of all the physical factors, light plays the best understood and most dramatic role. Many of the effects of light on plant growth are obvious and direct. Light energy is the driving force for photosynthesis, the series of chemical reactions in green plants in which carbon dioxide and water form carbohydrates and upon which all life ultimately depends. Insufficient light causes death or retardation of growth in green plants. But light also has indirect effects of great importance. Green plants possess small amounts of a pigment called phytochrome that can exist in two forms. One form absorbs red light (660 millimicrons, or mμ; 1 mμ = 3.937 × 10−8 inch). When plants containing this pigment absorb red light, the pigment is converted to another form, which absorbs far-red light (730 mμ); the latter form can be converted back again to the original red absorbing form. These conversions have dramatic consequences; for example, red light inhibits stem elongation and lateral root formation but stimulates leaf expansion, chloroplast development, red flower coloration, and spore germination. Cycles of red and far-red light also can affect flower formation.

The effects of light on animals, although less obvious, may be important, as, for example, the effect of light on growth of the reproductive system of some animals. Increase in day length, hence in the amount of light, seems to initiate growth and development of the sex organs (gonads) in some birds during the spring. Curiously, the eyes are not the receptors for the light signal that activates the endocrine system to initiate growth of gonads; rather, cells deep in the brain are sensitive to the small amounts of light that pass directly through the thin skull of the bird.

Most animals show cyclic activity, or rhythms, in various important physical (e.g., movement) and chemical (e.g., respiration) events that are essential to the individual. These rhythms are often regulated by short exposure to light.

Chemical factors

Chemical factors of importance in the environment include the gases in the atmosphere and the water, mineral, and nutritional content of food. Plants require carbon dioxide, water, and sunlight for photosynthesis; drought slows plant growth and may even kill the plant. The effects of atmospheric contaminants—e.g., oxides of nitrogen, hydrocarbons, and carbon monoxide—are known to have deleterious effects on the growth and reproduction of both plants and animals.

Plants and animals require minerals and small amounts of elements such as zinc, magnesium, and boron. Nitrogen and phosphorus are provided to plants as nitrates and phosphates in the soil. Inadequate quantities of any nutritional factor in the soil result in poor plant growth and poor crop yields. Animals require oxygen, water, and elements from the environment. Because they are unable to synthesize sugars from carbon dioxide, animals must acquire these nutrients through the diet, either directly, by the consumption of plants, or indirectly, by the consumption of other animals that in turn have utilized plants as food. If the quality or quantity of this food is poor, either growth is retarded or death occurs (see nutrition).

Vitamins, a class of compounds with a variety of chemical structures, are needed by animals in small amounts. Animals cannot synthesize all vitamins they require; those that cannot be synthesized must therefore be acquired in the diet, either from plants or from other animals that can synthesize the vitamin. Because certain vitamins are necessary in certain important metabolic reactions, vitamin deficiency during growth may have a variety of effects—stunting, malformation, disease, or death.

Internal factors

The organism is dependent on the environment for the raw materials for growth, but growth is also regulated internally. Because the size and form of plants and animals are under genetic control, events such as the rate and site of cell division and the extent of cell enlargement can be affected by mutations. It is not yet known, however, precisely how these factors, which are the ultimate determinants of growth, are controlled in individual cells.

One very important class of intrinsic growth regulators is that of the hormones. The principal plant hormone, auxin, is produced in the leaves; it moves by precise mechanisms, as yet poorly understood, to the other parts of the plant, controlling such processes as elongation of plant cells. Auxin somehow changes the characteristics of the rigid cell wall of the plant cell so that it becomes more flexible; the internal pressure within the cell then forces it to become larger. Other plant hormones may also play a role in the process; hormones such as cytokinins and gibberellins influence the rate of cell division in the meristems. Some dwarf plants can be stimulated to grow to normal size simply by applying gibberellin.

Hormones also play a decisive role in animal growth. One hormone from the pituitary gland at the base of the brain is called growth hormone because of its extensive and widespread effects on growth. A deficiency of growth hormone in pre-adolescents results in dwarfism, and oversupply of the hormone (often caused by a tumour) results in gigantism. If an excess of growth hormone is produced after the long bones can no longer grow—i.e., post-adolescence—a disease called acromegaly, which is characterized by increases in the size of the hands and feet and broadening of facial features, results. A deficiency of thyroid hormone in children also causes growth retardation.

The sex hormones secreted from the pituitary gland interact in a complex way to regulate the growth of the gonads. The gonads in turn produce estrogen and progesterone in females and testosterone in males; these hormones control the development of human secondary sexual characteristics—body hair, enlargement of mammary glands in females, and growth of the vocal cords in males. Although the growth hormones and sex hormones play a vital role in growth, the exact mechanism by which they function has not been established with certainty.

In addition to having the ability to synthesize the factors that regulate growth, plants and animals evidently possess exquisite mechanisms for integrating and regulating the production of hormones; i.e., the appropriate amounts of the right hormones are produced at the right time and the right place for normal growth.

Although many plants, including trees, grow throughout their lives, growth of parts of the organism is not perpetual; e.g., leaves of a given species attain a specific size and can grow no larger. In animals, growth stops entirely, except for replacement, after the juvenile period. The limits for both total body size and organ size are probably established by genetic mechanisms. The factors involved in limiting the growth of an organism are not yet definitely known, but evidence indicates that the liver releases into the bloodstream protein molecules that can limit growth of the organ. Thus, one theoretical view is that an organ may produce substances that serve to limit its own growth, thereby establishing a feedback mechanism. A protein called nerve-growth factor is important for the growth of some parts of the mammalian nervous system. If too much of the nerve-growth factor is present, growth of sympathetic nerve fibres is extensive and aberrant. If the nerve-growth factor is eliminated from the body—by injection of an antibody against the factor—the sympathetic nerves wither and disappear. Other subtle growth regulatory substances specific for various organ systems may eventually be discovered.

The dynamics of growth

Measurement of growth

The mathematical analysis of the rate of growth has been a subject of interest for many years. It is based on the rule of cell division: one cell gives rise to two daughter cells. Hence, the theoretical increase in cell number would be a geometric series, in which one cell produces two cells, then four, eight, 16, and so on. In reality, however, the rate of growth is not constant but declines after a period of time, usually because of influences in the environment or because of inherent genetic limitations. Thus the curve showing the growth of cell populations and of organisms is usually S-shaped, or sigmoid, when growth is plotted against time on a graph. The increase in cell number resulting from cell division accounts for the rising part of the curve; the rate of cell division decreases at the plateau in the curve. The S-shaped growth curve is generally applicable to the growth of organisms. If growth is plotted against time on a logarithmic scale, the early intense growth (called log growth) in the rising phase of the growth curve falls on a straight line.

The rate of growth may be defined by the differential equationv = dW/dt (1/W), in which v is the growth rate and W is the weight at any given time, t. The solution of this equation provides a value for relative increase—the increase in weight related to the initial mass of the growing substance. The animal that most closely approaches a constant rate of growth is an insect larva. In most animals the rate of growth declines as the organism becomes larger and older.

Although the S-shaped growth curve describes with fair accuracy the growth of populations of single cells, such as bacteria or cells of higher organisms in tissue culture—the growth in a sterile nutrient environment of cells of tissues from organisms—the growth rates of different parts of whole organisms vary. The relationship of the growth of one part of an organism to that in another part is called allometry. An equation expressing the fundamental relationship of allometric growth is y = bxk in which y is the size of one organ; x is the size of another; b is a constant; and k is known as the growth ratio. Although such mathematical tools have allowed a very thorough description of the differential growth of different parts of an organism, they have unfortunately not provided insight into the physical and chemical control of the growth rate.

The study of growth

Even though the chemical, physical, and genetic bases of growth are elusive, much has been learned about the process by growing tissues in a sterile nutrient environment. Even if the source of the tissue is an organ that has completely stopped growing, such as the nervous system of an animal or the phloem of a plant, the cells will begin to grow again in culture, often at a logarithmic rate of increase. It may therefore be concluded that the organism as a whole places constraints upon the ability of individual cells to reproduce and that, when these constraints are removed, the growth potential of the cells is no longer restrained. Even in tissue culture, however, the rate of cell growth eventually slows, hence the sigmoid-shaped growth curve. During the rapid growth phase of cells in tissue culture, they usually lose the ability to carry out the specialized function characteristic of their organ of origin; for example, if cartilage cells divide rapidly, they no longer synthesize cartilaginous matrix. This phenomenon of apparent despecialization has been a topic of great theoretical interest: are rapid growth and specialization mutually exclusive activities? Evidence shows that some types of specialized cells may be maintained in tissue culture for very long periods of time and still retain the ability to carry out specialized biosyntheses, so that the apparent loss of specialized function in tissue culture cells may not fundamentally result from a mutual exclusivity of growth and differentiation.

When the growth of tissue-culture cells begins to slow, one factor responsible is exhaustion of critical components from the medium. But even if the medium is frequently replaced, when the bottom of the culture dish becomes densely packed with a layer of cells, the growth rate drops—a phenomenon called contact inhibition of growth. It is believed that cells so close that they are always touching provide a signal that retards the rate of cell division. Apparently identical cells in tissue culture also show great variation in growth rate. Some cells from the skin, for instance, when placed in culture, may divide every eight hours; other similar cells may divide only every 36 hours. The growth of cells in a controlled environment such as tissue culture offers many possibilities for studying the fundamental mechanisms controlling cell growth and, consequently, the growth of organisms and populations.